bar
is a image created just with rbd, you can see that all the cinder volumes
starts with ‘ volume-<uuid>
‘.

Create snapshot

So for rbd everything is thinly provisioned, and snapshots and clones use copy-on-write
.

When you have a copy-on-write ( cow
) snapshot, it means it has a depency on the parent that it was created from. The only unique blocks in the snapshot or clone will be the blocks that have been modified (and thus copied) that makes snapshot creation very very fast
(you only need to update metadata) and data doesn’t actually move or copy anywhere ….
instead it’s copied on demand, or on write !

This dependency means that you cannot, e.g. delete a volume that has snapshots

because that would make those snapshots unusable, like pulling the rug out from under them.